Semiconductors and other electrical components generate heat as a by-product of their operation. As technology has advanced, the amount of heat to be dissipated from many of these components has risen dramatically while the acceptable cost of heat dissipating devices has remained constant or, in many cases, has dropped. Consequently, the art of heat sinking to cool heat-dissipating components has continually evolved to meet these new market requirements.
Finned metal heat sinks have traditionally been utilized to dissipate heat generated by electrical components. These finned metal heat sinks typically include a substantially rectangular base unit to which the heat generating electronic devices are mounted, and a plurality of fins projecting from the base unit for dissipating the generated heat. In many applications, a fan is utilized to force cooling air across the fins of the heat sink in order to enhance cooling from the heat sinks. In these applications, the amount of heat that may be dissipated from the heat sink at a given air velocity is directly related to the surface area exposed to the airflow. Thus, heat sink designers have striven to maximize the surface area of the fins in order to provide optimum heat transfer from the heat sink to the surrounding atmosphere.
Heat sinks have traditionally been fabricated by extruding aluminum through a die, which is cut to the required shape specifications such that the base unit and fins are of integral construction. Commercial extrusion processes have typically been limited to aspect ratios, defined as the ratio of fin height to spacing between fins, in the range of 16:1, effectively limiting the achievable surface area for a given volume heat sink. However, the surface area requirements in many applications have exceeded the maximum achievable surface area for extruded heat sinks. Thus, there has been a need for an alternative method of fabricating heat sinks that will allow these increased surface area requirements to be met.
One common alternative method has been to extrude a base plate with a plurality of grooves and to bond sheet metal fins into these grooves via a thermally conductive epoxy. This method allows heat sinks having high fin densities to be fabricated, but has a number of significant drawbacks. First, both the base and fins must be meticulously cleaned and etched prior to bonding in order to obtain a strong bonded joint between the parts. Second, even epoxies heavily loaded with conductive materials such as aluminum oxide do not possess the thermal conductivity of a metal to metal joint, resulting in significant conduction losses through the base to the fins. Third, epoxies are not electrically conductive and, therefore, the finished heat sinks may not be effectively anodized, as the electricity required for the anodization process cannot pass through the base to the fins. Finally, the epoxy bonding process requires that fins be inserted within the grooves in the extrusion and carefully fixtured prior to curing, adding significantly to the overall cost of these heat sinks.
Another method involves the use of a horizontal milling machine to form a finned heat sink by milling a plurality of parallel slots in a solid piece of metal to form a plurality of fins. This method overcomes many of the drawbacks of bonded fin heat sinks, but has drawbacks of its own. First, this process wastes significant amounts of aluminum, resulting in significant scrap costs. Second, tooling costs for such a process are high as milling inserts must be replaced on a regular basis. Finally, this process often results in significant metal burrs, which must be removed in a secondary operation after milling.
A number of patents have sought to address the need for an alternative heat sink fabrication method. For example, U.S. Pat. No. 3,216,496, issued to Katz, discloses a base unit having a plurality of slots for receiving separately fabricated fins. The slots extend transversely the length of the bar and are arranged so as to hold the fins in substantially parallel relationship. The fins are joined to the base by inserting the fins into the slots and swaging the material between adjoining slots into intimate contact with the fins utilizing a knife edged tool attached to a high pressure hydraulic press. Unfortunately, the method of Katz utilizes a tool extending along the entire length of the fin and, thus, requires a large amount of force to be applied to the tool. As this is the case, large, noisy, and expensive presses must be utilized to perform the task. Further, this process has been known to lift the fins upwardly from the grooves, leaving an air space at the bottom of the grooves in the bus bar, which allows for air and moisture to enter resulting in corrosion and reducing the thermal contact between the base unit and fins.
U.S. Pat. No. 5,638,715, issued to Lipinski, discloses a method for fabricating high fin density heat sinks from a plurality of fins, each having a bottom portion of generally bell-bottom shape, and a base unit having a plurality of mating dovetail features by utilizing a plurality of rollers to apply pressure on opposite sides of the fin for providing downward and inward swaging against a dovetail joint. The method includes the steps of placing the plurality of fins loosely in respective ones of the grooves, and applying downward and horizontal pressure to the base unit intermediate respective ones of the fins for swaging the base unit and urging the fins downwardly into the respective ones of the grooves.
The method of Lipinski results in heat sinks having increased aspect ratios over what is commercially available. However, there are a number of drawbacks to this method of fabrication. First, the requirement that the bottom of each fin be bell shaped requires that extruded fins be utilized. This requirement increases the cost over what can be achieved utilizing sheet aluminum and decreases the ability of the designer to customize the height of the fins to the exact requirements of the user, as can be accomplished by slitting sheet aluminum fins. Second, the mating dovetail details on the base unit require that the base plates by extruded as well, again reducing the flexibility of designers. In addition, the arrangement of the extruded dovetail details is also limited by the extricable limits on aspect ratios, limiting the minimum fin spacing and creating yet another limitation on achievable surface area in a given volume. Finally, the compounding of extrusion tolerances between the fins and base results in the same discontinuities in fin to base joints as were found in Katz and, as in Katz, allowing air and moisture to enter resulting in corrosion and reducing the thermal contact between the base unit and fins.
Yet another known process involves the staking of fins into a plurality of parallel mating details in a heat sink base. This process operates in a manner similar to a common stapler, with a dual pronged end of an extruded fin being forced into the heat sink base at a force sufficient to deform the end, attaching the fin to the base unit. As was the case with the method of Lipinski, both the fin and base must be extruded, resulting in the same drawbacks. Further, as this process does not utilize a dovetail joint, the fins have tendency to loosen if subjected to vibration or corrosive environments, resulting in a significant reduction of thermal contact between the base unit and fins. Finally, as was the case with the process of Katz, this process requires that a significant amount of force be applied to the fins and, consequently, large, noisy, and expensive presses must be utilized to perform the task.
In addition to finned heat sinks, a number of designers have turned to heat sinks utilizing a plurality of pins extending from a base plate. These pin fin heat sinks have traditionally been manufactured either by casting or by extruding a finned heat sink and cross cutting the fins to create pins. Although these processes allow the surface area of a heat sink to be increased over extruded fin type heat sinks, these methods also have significant drawbacks. Both cast and cross cut pins are limited in their spacing by the limitations of the casting and extrusion processes, respectively. In addition, each requires a significant investment in tooling to create new parts, essentially limiting design variability and heat sink customization.
Accordingly, there is a need for a heat sink, and a method of fabricating a heat sink, that allows high aspect ratios to be achieved, does not require the use of large, noisy, and expensive presses to join fins to base units, does not suffer from significant conduction losses through the base to the fins, does not tend to lift fins from their respective base units, does not create discontinuities in the fin to base joints that promote corrosion and decrease thermal contact, does not require the use of extruded fins or base plates, and does not require a large initial investment in equipment.